1. Field of the Invention
This invention relates to volumetric flowmeters, and more particularly concerns acoustic displacement flowmeters for determining flow rate of fluids (e.g., gases) commonly used in chromatography.
2. Description of the Prior Art
Volumetric flow measurement devices 10 exist that employ acoustic displacement transducer (ADT) technology. In such devices 10, gas is passed through a channel 20 in the device 10 which includes a precision orifice restriction, a reservoir or chamber containing an acoustic displacement transducer (ADT), and a normally open low dead volume valve 14. FIG. 1 illustrates the flow path in such volumetric flow measurement devices 10 under flow conditions.
FIG. 2 illustrates current flowmeter technology. Related commercially available products include the Agilent ADM1000, the ADM2000, and the ADM3000 flowmeters. Patents relating to current flowmeter technology include U.S. Pat. Nos. 5,460,038, 5,540,104, 5,723,783, European Patent EP 0 553 550, and European Patent EP 0 672 893, all of which are incorporated herein by reference. As shown in FIG. 2, the probe gas inlet 11 is affixed to a gas source or vacuum source, not shown (the system is also equipped to measure vacuum, or negative flow). The gas flows through the device 10 with the first restriction being the precision orifice 12. A measurement is made when the valve 14 closes and the gas, instead of flowing from the valve 14 to the exhaust 15, flows from the flow path upstream of the valve 14 into the ADT chamber 13a of the acoustic displacement transducer 13. In this device 10, the ADT sensor is a conventional audio speaker. When the valve 14 shuts, the chamber 13a containing the speaker “inflates” and moves the speaker cone position. The gas flow rate measurement relates to the rate of deflection of the speaker. After the measurement is made, the valve 14 opens and the gas vents through exhaust 15. In currently known devices 10, the exhaust 15 resides inside the flowmeter enclosure or housing 16 and may eventually exit the device 10 through holes 17 found in the enclosure or housing 16.
Current flowmeter technology vents the gas stream from the valve 14 directly into the device 10, thereby exposing the gas to all electronic and mechanical components resident in the device 10. In cases where non-explosive gases are employed (e.g., helium), it is not necessary to isolate the gas from potential ignition sources (e.g., electronic and mechanical components) that reside inside the device 10. However, in cases where the flow measurement relates to an explosive gas (e.g., hydrogen), there exists a danger of explosion due to such explosive gas coming into contact with potential ignition sources (e.g., electronic and mechanical components) that reside within the device 10.